The present invention relates generally to gas generators used to inflate vehicle airbags, and specifically, to the autoignition of gas generating or ignition enhancer materials.
Inflatable occupants restraint systems have been developed to protect vehicle occupants in the event of a crash by rapidly inflating a cushion or bag between a vehicle occupant and the interior of the vehicle. An inflated airbag reduces the injury sustained by a vehicle occupant during a crash by having the vehicle occupant collide with an inflatable cushion rather than the hard surfaces of the interior of the vehicle. The inflatable cushion or airbag absorbs much of the vehicle occupant""s energy during a crash to provide the vehicle occupant with a gradual and controlled deceleration during a crash.
A typical method for inflating an airbag is with a gas generator that generates gas by an exothermic reaction of a fuel and an oxidizer. During a crash of a significant magnitude an electrical signal from one or more crash sensing devices is sent to an inflator igniter. The initiation of the igniter starts a string of chemical reactions in the inflator, in which gas for inflating the airbag is rapidly produced.
The inflator may be subjected to abnormally high temperatures, for example if the inflator is involved in a fire during shipment. During a vehicle fire, the strength and structural integrity of the housing will be diminished. As the temperature increases, the burn rate for the pyrotechnic material increases which will cause the chamber pressure to also increase. Moreover, at elevated temperatures, the gas generant can melt or decompose and become unstable which can result in loss of ballistic control resulting in over pressurization. Therefore, if a vehicle is involved in a fire, the ignition of the gas generant composition can lead to overpressurization and loss of ballistic control, which could result in structural failure of the inflator housing.
To overcome the potentially dangerous situation of housing failure, autoignition materials are used which spontaneously combust or ignite at a temperature lower than that which would lead to the failure of the inflator housing due to over pressurization. When the autoignition material spontaneously ignites, the generated heat ignites the gas generant or ignites a booster material, and the inflator functions normally. Thus, the gas generating material is ignited when a preselected temperature is reached, which is below the temperature that would cause the gas generating material to behave in an unpredictable manner.
The number of autoignition compositions available in the prior art is limited, and in part includes nitrocellulose and mixtures of potassium chlorate and sugar. Smokeless powder or gunpowders are nitrocellulose based substances that autoignite at approximately 177xc2x0 C. (≈350xc2x0 F.) A problem with using gunpowders for autoignition materials is that these materials are granular and need to be packaged before being placed in the inflator. The packaging of the nitrocellulose-based material into an autoignition container complicates the manufacturing process by adding an additional sub-process step.
U.S. Pat. No. 4,561,675 teaches an autoignition material located in a container adjacent to the inflator. The auto-ignition material is a smokeless powder that ignites at approximately 176xc2x0 C. (350xc2x0 F.). In the preferred embodiment, the smokeless rifle powder is IMR 4895, which is largely composed of nitrocellulose.
U.S. Pat. No. 4,858,951 teaches small grains of an autoignition material physically mixed with a booster material, such that at a predetermined temperature, the autoignition material will spontaneously ignite. This event will cause the booster material to ignite which will in turn ignite the gas generant. The preferred autoignition material is smokeless powder, and the booster material is a mixture of boron potassium nitrate, titanium hydride, and potassium perchlorate.
The present invention is directed to an autoignition material for igniting a gas generant at a preselected temperature. The autoignition material comprises 70%-95% by weight nitrocellulose and 5%-30% by weight inert plasticizer and is not smokeless powder. Upon exposure of the inflator housing to excessive heat, the autoignition material spontaneously ignites the booster material or gas generant, which in turn safely deploys the inflator.
An advantage of the present invention is that the autoignition material is a monolithic grain, rather than a granular material. Utilizing a monolithic autoignition material is a significant reduction in manufacturing cost since a subassembly of an autoignition material is not required. The monolithic grain in the claimed invention does not need to be packaged and can be directly installed in the inflator housing.